1. Field of the Invention
The present invention relates to heat-treating oil compositions, and more particularly to heat-treating oil compositions for quenching a metal material which hardly cause fluctuation in hardness or quenching distortion of the treated metal material even when a large number of the metal materials are quenched therewith at the same time.
2. Description of Related Arts
Metal materials such as steel materials are subjected to various heat treatments such as quenching (hardening), tempering, annealing and normalizing in order to improve properties thereof. Among these heat treatments, in the quenching treatment, for example, a heated steel material having an austenite structure is cooled at an upper critical cooling rate or more to transform the austenite structure into a hardened structure such as martensite. The steel material subjected to the quenching treatment has a very high hardness. In the quenching treatment, as a coolant, there have been generally used oil-based, water-based (aqueous solution-based) or emulsion-based heat-treating liquids. The quenching treatment for the steel material is explained below. When the heated steel material is put into the heat-treating liquid as the coolant, the cooling rate of the steel material is not kept constant, and usually varies via the following three stages. That is, the steel material is cooled through (1) the first stage (vapor blanket stage) in which the steel material is enclosed with a vapor blanket (film) of the heat-treating liquid, (2) the second stage (boiling stage) in which the vapor blanket is broken and the heat-treating liquid is boiled, and (3) the third stage (convection stage) in which the temperature of the steel material is decreased to a temperature lower than a boiling point of the heat-treating liquid so that heat is removed from the steel material by convection of the heat-treating liquid. Among these three cooling stages, the cooling rate of the second boiling stage is largest. The conventional heat-treating oils exhibit a rapid rise-up of thermal transmission showing a cooling power thereof, in particular, in the boiling stage, so that the material to be treated with the oils tends to undergo a very large temperature difference on a surface thereof under the transition condition between the vapor blanket stage and the boiling stage. With such a temperature difference, the material tends to suffer from thermal stress or transformation stress due to difference in heat shrinkage rate or transformation time between both the cooling stages, resulting in increase in quenching distortion thereof.
Upon the heat treatment of metals, in particular, upon quenching treatment thereof, it is important to select an appropriate heat-treating oil suitably used under the intended heat-treating conditions. The selection of inappropriate heat-treating oils tends to fail to impart a sufficient hardness to the material quenched, or tends to generate considerable distortion therein.
The heat-treating oils are generally classified into Types 1 to 3 according to JIS K2242. Among them, the heat-treating oils used for the quenching treatment include #1 and #2 oils of Type 1 and #1 and #2 oils of Type 2. In JIS K2242, as a measure of the cooling power of oils, it is prescribed that a cooling time (s) required for cooling a metal material from 800° C. to 400° C. when measured on the JIS cooling curve is 4.0 s or shorter for Type 1 #2 oil, 5.0 s or shorter for Type 2#1 oil, and 6.0 s or shorter for Type 2#2 oil. The shorter cooling time means the higher cooling power and, therefore, results in higher hardness of the heat-treated material. In general, the hardness and quenching distortion of the heat-treated material have a so-called trade-off relation to each other, i.e., the higher the hardness, the larger the quenching distortion becomes.
In addition, as an industrial index of a cooling power of quenching oils, there has been extensively used the H value which has been frequently described in catalogues, etc., distributed by the respective oil makers and used as a measure showing the cooling power of the quenching oils. The H value of the quenching oil is calculated from a cooling time required for cooling the metal material treated with the oil from 800° C. to 300° C. which is measured on the cooling curve prepared according to JIS K2242, and has been widely used to show the cooling power of the oil. The users can select a suitable quenching oil on the basis of the H value as an index to attain the aimed degrees of hardness and quenching distortion of the material to be treated. For example, the JIS Type 2 #1 oil has been extensively used for quenching gear parts for automobiles which tend to be adversely influenced by distortion generated therein. This is because the gear parts treated with the JIS Type 1 oils show not only a too large distortion but also a too high hardness in some kinds of the gear parts, whereas the gear parts treated with the JIS Type 2 #2 oil tend to lack in hardness notwithstanding a small distortion thereof.
Meanwhile, most of the parts for automobiles such as speed change gears and reduction gears are mass-produced, and a large number of these parts are stacked in one tray and subjected to quenching treatment at the same time, i.e., a so-called collective quenching. In such a collective quenching, the stacked parts to be quenched tend to undergo fluctuation in hardness or distortion due to the difference in positions in the tray. For example, upon the collective quenching, the parts set in a lower position of the tray tend to show a higher hardness, whereas those set in an upper position of the tray tend to show a lower hardness.
In order to prevent the parts treated in the stacked state from undergoing fluctuation in hardness or distortion thereof upon the collective quenching, there has been proposed the use of additional special devices such as vibrators and injectors (for example, refer to claims of Japanese Patent Application Laid-open No. 286517/2003). However, the use of such additional devices in the conventional apparatuses leads to high costs, and further it has been difficult to modify the conventional apparatuses when applying some kinds of devices thereto. Therefore, it has been demanded to develop techniques for preventing the above fluctuation in hardness or distortion of the parts treated with the quenching oils only by the effect of these oils without need of any additional investments for facilities.
Further, in the literature “Heat Treatment”, Vol. 43, No. 2, pp. 93 to 98, it is described that when a material is treated with two kinds of base oils which have the same viscosity but are different in a 5% distillation temperature from each other (i.e., one base oil has a 5% distillation temperature of 350° C. or lower and the other has a 5% distillation temperature of more than 350° C.) to evaluate the hardness and distortion thereof, the material treated with the base oil having a 5% distillation temperature of 350° C. or lower shows a smaller distortion while maintaining a high hardness (refer to FIGS. 12 and 13 of the literature). However, the techniques described in the above literature have the following problems.
One of the problems resides in that the distortion is evaluated by warpage of the SUJ2 shaft part. The cooling process using the heat-treating oil proceeds through the vapor blanket stage, boiling stage and convection stage as described above. In the case of the parts having such a shaft shape, it is known that the distortion thereof is considerably influenced by change in vapor blanket breaking time in the vapor blanket stage and, therefore, the influence of a vapor blanket retention time (characteristic time (s)) rather than viscosity or boiling point of the heat-treating oil is more dominant for distortion of the parts treated therewith. Although no vapor blanket retention time is specified in the above literature, it is easily suggested from composition of the base oil used therein that the shorter vapor blanket retention time results in a smaller distortion of the parts treated therewith, since it is an ordinary tendency. Also, in the above literature, the distortion is evaluated using the SUJ2 part, whereas the hardness is evaluated using the S45C part, i.e., the two properties are evaluated using different materials from each other. For the purpose of obtaining such a heat-treating oil satisfying the requirements of both hardness and distortion, it is required to evaluate the hardness and distortion using the same material. If the distortion is evaluated with respect to the S45C part used for evaluating the hardness, it is expected that substantially no change in distortion between before and after the quenching treatment is observed due to poor quenching property thereof.
The other problem encountered in the above literature resides in that the oils studied therein have a relatively high cooling power close to that of the JIS Type 1 #2 oil, and such oils having a high cooling power are not usually used for the heat treatment of parts which tend to be adversely influenced by distortion generated therein. In general, the parts which tend to be adversely influenced by distortion generated therein are frequently treated with the heat-treating oils having a low cooling power which is capable of preventing these parts from undergoing distortion, such as JIS Type 2 #1 oil and, in some cases, JIS Type 2 #2 oil. For example, gears for automobiles have been extensively heat-treated with the JIS Type 2 #1 oil. Under these circumstances, in order to evaluate the distortion, the materials such as SCM420 and SCr420 which have been widely used for the parts for automobiles such as speed change gears, transmissions and reduction gears are preferably heat-treated with the JIS Type 2 #1 oil.
The present inventors have already proposed the heat-treating oil composition capable of not only preventing a metal material from undergoing cooling unevenness when quenched therewith to ensure an adequate hardness of the quenched metal material, but also reducing the quenching distortion generated therein, which contains a mixed base oil composed of a low-viscosity base oil having a kinematic viscosity at 40° C. of 5 to 60 mm2/s and a high-viscosity base oil having a kinematic viscosity at 40° C. of 300 mm2/s or higher (refer to claims of Japanese Patent Application Laid-open No. 327191/2002). However, according to the subsequent studies made by the present inventors, it has been found that when the heat-treating oil compositions containing the low-viscosity base oil in an amount of 50% by weight or more as described in Examples of the above Japanese Patent Application are used for quenching the parts such as gears for automobiles, the thus treated parts show a too high hardness.